Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.
In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance, and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues. All capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor.
Due to the geometries involved in a typical fingerprint pattern, the difference in capacitance between a ridge and a valley can be considered to be small. Further, different materials are typically needed between the capacitive sensing elements and the finger. Such materials increases the distance between the finger and the sensing elements and will therefore inevitably further decrease the sensed difference in capacitance between a ridge and a valley. For example, some implementations of capacitive fingerprint sensors in mobile devices demand that the fingerprint sensor is covered by what can be considered very thick materials, such as e.g. a cover glass in a mobile phone. Such implementations will drastically reduce the dynamic range of the sensor.
When further increasing the distance between the sensing elements and the finger, the difference in the capacitive coupling between a ridge and valley quickly reaches a magnitude that is similar or even smaller than the system noise floor of the sensing device. In order to increase sensing performance it is therefore desirable to lower the noise floor.
Accordingly, when the signal-to-noise ratio cannot be improved further by changing the structure or sizing of the signal amplification and readout without significant cost, it's desirable to improve the conditions at the sensing element.